I was wondering if you had trimmed.
We have 5 horses, I trimmed 2 and a few days later those 2 had sores around nose and mouth, where I had trimmed. Was told that sores were due to a close trim and heavy dew on grass.
I just kept sores clean and applied thick amounts of ointment to keep sores from splitting and give relief, the sores quickly disappeared. I never had any problems before, but I'm very careful with clippers now. I keep clippers clean after each use, and have new blades handy......no more problems here.
The sores around my horse's mouth did not spread.
I also have a bald face paint and his nose tends to sunburn, I keep sunblock on him because he rips his face mask off and drops it in the pond. Putting on sunblock is a lot better than taking a dip in a nasty pond to get his face mask.
Hope you figure it out....